附加到二维字符串切片时的奇怪行为

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• Golang slice reference confusion 3 answers

I'm attempting to implement Heap's Permutation Algorithm in Go. It's supposed to return all the possible permutations given an input set.

func Permute(in []string) [][]string {
    c := make([]int, len(in))
    out := make([][]string, 0)
    for i := range out {
        out[i] = make([]string, 0)
    }
    fmt.Println(in)
    out = append(out, in)
    i := 0
    for i < len(in) {
        if c[i] < i {
            if i%2 == 0 {
                in[0], in[i] = in[i], in[0]
            } else {
                in[c[i]], in[i] = in[i], in[c[i]]
            }
            fmt.Println(in)
            out = append(out, in)
            c[i]++
            i = 0
        } else {
            c[i] = 0
            i++
        }
    }
    return out
}

The Println statements display the expected outputs. The out value is returned with the correct number of entries, but the entries are all the same. Please help me understand what I am missing.

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Answer 1

The value of in is never changed, so every element in out has that same value. in contains the slice header, which is a pointer to the underlying array, a length, and a capacity. You're changing the values in the array, but the slice header remains the same. I think you want to make a copy of in inside the for loop so you're operating on a different array every time:

for i < len(in) {
    inCopy := make([]string, len(in))
    copy(inCopy, in)
    in = inCopy
    // ...
}

See https://blog.golang.org/go-slices-usage-and-internals for an explanation of how slices work.